1. Field of the Invention
The present invention relates to a laminated heat exchanger that is employed in an air conditioning system or the like mounted in vehicles, and is constituted by laminating tube elements, each provided with tank portions and a passage portion, alternately with fins over a plurality of levels.
2. Description of the Related Art
In the prior art, tube elements in a so-called drawn cup type laminated heat exchanger are each constituted by bonding two formed plates face-to-face and are each provided with tank portions where heat exchanging medium collects and is distributed and a passage portion communicating with the tank portions. The passage portion is provided with. a number of beads formed therein for promoting heat exchange. Furthermore, shoal-like beads are formed in the areas where the tank portions change or transition into the passage portion. In addition, intake/outlet portions, which project out and open from tank portions so as to be connected with piping, are formed in specific tube elements.
However, a heat exchanger structured as described above has problems in that, since the heat exchanging medium flows in and out through piping connected at specific tank portions, the passage resistance is reduced in the tube elements where the intake/outlet portions are formed by an amount corresponding to the quantity of heat exchanging medium that does not travel through the other tank portions. Also, depending upon the type of heat exchanger, these tube elements may constitute the shortest path and, in particular, when the flow rate is low, the flow concentrates in the tube elements provided with the intake/outlet portions, which adversely affects the temperature distribution in the heat exchanger.
For instance, in the case of a unilateral tank type evaporator which is known in the prior art, it has been confirmed that, as shown in FIG. 4A, the surface temperature at the tube elements provided with the intake/outlet portions, which is indicated by the filled circles, is lower than the temperature at the other tube elements, with the temperature becoming higher in the tube elements further away from the intake/outlet portions. This results in an increase in the difference (.DELTA.T) in the surface temperature between the tube elements where the temperature is the highest and the tube elements where the temperature is the lowest (the tube elements provided with the intake/outlet portions in the prior art).
In addition, a unilateral tank type laminated heat exchanger which, in order to improve heat exchanging performance, is achieved by reducing the inconsistency in the air temperature distribution of the air passing through the heat exchanger in the prior art is disclosed in Japanese Unexamined Patent Publication No. S 63-3153.
In this laminated heat exchanger, which is constituted by laminating passage units (tube elements) alternately with corrugated fins over a plurality of levels, the passage units are each provided with a pair of tanks, i.e., a first tank and a second tank at one side, with a U-shaped coolant passage (U-shaped passage) communicating between the pair of tanks and a first communicating hole (communicating hole) or a second communicating hole (communicating hole) at each tank. Thus, when the tanks in adjacent passage units are bonded together, two tank groups are formed extending in the direction of the lamination (a first tank group and a fourth tank group, a second tank group and a third tank group). The first tank group and the fourth tank group are partitioned in the middle so that they do not communicate with each other. An intake pipe is mounted at the first tank group and an outlet pipe is mounted at the fourth tank group. In addition, in the third tank group, one or two passage units provided with a constricting portion having a constricting hole with a diameter smaller than that of the second communicating hole is provided to partially reduce the flow passage area for the coolant.
According to the publication, the constricting portion formed within the third tank group prevents the liquid coolant flowing within the third tank group from flowing fast. As a result, the liquid coolant is prevented from flowing far into the third tank group in a great quantity, which, in turn, causes the liquid coolant to flow in ample quantity into the passage units communicating with tanks in the middle and toward the frontmost area among the tanks constituting the third tank group, thereby achieving consistency in the quantity of the liquid coolant flowing throughout.
However, if the flow rate of heat exchanging medium in a liquid form is restricted simply by providing a constricting portion in a specific tank group, as in the prior art heat exchanger described above, when the heat exchanging medium in a liquid form is flowing at a low flow rate, it will be inhibited more than necessary by the constricting portion, thus preventing the heat exchanging medium in a liquid form from being thoroughly distributed throughout the tanks further inward relative to the intake/outlet portions in the tank groups provided with the intake/outlet portions, so as to adversely affect the temperature distribution even more.